Transmission system with adaptive channel encoder and decoder

ABSTRACT

In a transmission system comprising a transmitter ( 4 ) coupled via a transmission channel ( 10 ) to a receiver ( 6 ). The transmitter ( 4 ) comprises a channel encoder ( 14 ) for deriving encoded symbols from source symbols. The receiver ( 6 ) comprises a channel decoder ( 28 ) for reconstructing the source symbols from the signal received from the transmission channel ( 10 ).  
     According to the present invention, the transmitter ( 4 ) comprises a separate encoder for coding and transmitting a coding property used in the channel encoder ( 14 ) to the receiver ( 6 ). The receiver ( 6 ) is arranged for receiving the encoded coding property from the transmission medium, and the separate channel decoder ( 26 ) is arranged for decoding the encoded coding property. The coding property provided by the separate channel decoder ( 26 ) is passed to setting means ( 27 ) in the channel decoder ( 28 ) for setting the coding property of the channel decoder ( 28 ).

[0001] The present invention relates to a transmission system comprisinga transmitter coupled via a transmission channel to a receiver, in whichthe transmitter comprises a channel encoder for encoding source symbolsinto coded symbols, and in which the receiver comprises a source decoderfor deriving reconstructed source symbols from the coded symbolsreceived from the transmission channel, the transmission systemcomprising coding property setting means for setting at least one codingproperty of the channel encoder and the channel decoder.

[0002] The present invention also relates to a transmitter and areceiver for use in such a transmission system. The invention furtherrelates to a transmission method.

[0003] Such transmission systems can be used in applications where thequality of the transmission channel shows considerable variations. Toenable a virtual error free transmission over such a transmissionchannel, in the transmitter the source symbols are encoded using achannel encoder according a code which has error correcting and/or errordetecting capabilities. In the receiver the source symbols arereconstructed by a channel decoder. Useful codes can includeconvolutional codes and several types of block codes such asReed-Solomon codes. Also a combination of a convolutional code with ablock code is often used.

[0004] The ratio between the number of source symbols and the number ofchannel symbols of such a code is called the rate of the code. The errorcorrection capabilities of such a code depend heavily on the rate of thecode. In case of a transmission channel with a strongly varyingtransmission quality the rate of the used channel code should be chosento obtain virtually error free transmission at the worst channelconditions. This leads to a loss of useful transmission capacity whenthe transmission quality is high.

[0005] To prevent this loss of transmission capacity, the transmissionsystem can set the at least one coding property e.g. the rate of thechannel encoder, in dependence on the transmission quality. It isfurther observed that it can be desirable to set the coding property ofthe channel encoder and decoder decoder in dependence of the type ofsource symbols to be transmitted. E.g. the transmission of data signalsrepresenting computer files, requires bit error rates below 10⁻¹⁰, andthe transmission of digitized speech signal may require bit error ratesonly below 10⁻⁴.

[0006] The problem to be solved by the present invention is how toinform the receiver of a change of the at least one property of thechannel encoder, in order to be able to decode it.

[0007] To solve this problem the transmission system according to thepresent invention is characterized in that the transmitter comprises afurther channel encoder for obtaining an encoded coding property fromthe at least one coding property, in that the transmitter is arrangedfor transmitting the encoded coding property to the receiver, in thatthe receiver comprises a further channel decoder for obtaining the atleast one coding property from the encoded coding property, and in thatthe receiver is arranged for setting the at least one coding property ofthe channel decoder according to the at least one coding property.

[0008] By using a separate channel for transmitting the at least onecoding property, it is obtained that the transmission of this codingproperty does not depend on the (main) channel coding scheme which mightfail under rapidly changing channel conditions. Because in general theamount of information needed to communicate the at least one codingproperty is very small, a very strong error correcting coding scheme canbe used in the further channel encoder.

[0009] An embodiment of the invention is characterized in that that thefurther channel encoder comprises a block encoder, and in that thefurther channel decoder comprises a block decoder.

[0010] The use of a combination of a block encoder and a block decoderenables an easy decoding, in particular when the amount of informationto be transmitted is low.

[0011] A further embodiment of the present invention is characterized inthat the encoded symbols are arranged in frames, and in that thetransmitter is arranged for splitting the encoded coding property intoat least two parts, and for transmitting said at least two parts insubsequent frames.

[0012] By splitting the encoded coding property in at least two partswhich are transmitted in subsequent frames, it becomes possible tochoose longer codewords in order to obtain a further increased errorcorrecting capability.

[0013] A still further embodiment of the present invention ischaracterized in that the transmission system comprises transmissionquality determining means for deriving a transmission quality measurefrom the channel decoder in the receiver, and means for transmitting viaa further transmission channel the quality measure to the transmitter.

[0014] By using a return link from the receiver to the transmitter, itbecomes easy to obtain a transmission quality at the transmitter. In asimilar way, it is also possible to use transmission quality dependentchannel encoding on a full duplex link.

[0015] The present invention will now be explained with reference to thedrawing figures.

[0016]FIG. 1 shows a transmission system according to the invention.

[0017]FIG. 2 shows a frame structure use in the transmission systemaccording to FIG. 1.

[0018] The transmission system according to FIG. 1, comprises threeimportant elements being the TRAU (Transcoder and Rate Adapter Unit) 2,the BTS (Base Transceiver Station) 4 and the Mobile Station 6. The TRAU2 is coupled to the BTS 4 via the A-bis interface 8. The BTS 4 iscoupled to the Mobile Unit 6 via an Air Interface 10.

[0019] A main signal being here a speech signal to be transmitted to theMobile Unit 6, is applied to a speech encoder 12. A first output of thespeech encoder 12 carrying an encoded speech signal, also referred to assource symbols, is coupled to a channel encoder 14 via the A-bisinterface 8. A second output of the speech encoder 12, carrying abackground noise level indicator B_(D) is coupled to an input of asystem controller 16. A first output of the system controller 16carrying a coding property, being here a downlink rate assignment signalR_(D) is coupled to the speech encoder 12 and, via the A-bis interface,to coding property setting means 15 in the channel encoder 14 and to afurther channel encoder being here a block coder 18. A second output ofthe system controller 16 carrying an uplink rate assignment signal R_(U)is coupled to a second input of the channel encoder 14. The two-bit rateassignment signal R_(U) is transmitted bit by bit over two subsequentframes. The rate assignment signals R_(D) and R_(U) constitute a requestto operate the downlink and the uplink transmission system on a codingproperty represented by R_(D) and R_(U) respectively.

[0020] It is observed that the value of R_(D) transmitted to the mobilestation 6 can be overruled by the coding property sequencing means 13which can force a predetermined sequence of coding properties, asrepresented by the rate assignment signal R_(U), onto the block encoder18 the channel encoder 14 and the speech encoder 13. This predeterminedsequence can be used for conveying additional information to the mobilestation 6, without needing additional space in the transmission frame.It is possible that more than one predetermined sequence of codingproperties is used. Each of the predetermined sequences of codingproperties corresponds to a different auxiliary signal value.

[0021] The system controller 16 receives from the A-bis interfacequality measures Q_(U) and Q_(D) indicating the quality of the airinterface 10 (radio channel) for the uplink and the downlink. Thequality measure Q_(U) is compared with a plurality of threshold levels,and the result of this comparison is used by the system controller 16 todivide the available channel capacity between the speech encoder 36 andthe channel encoder 38 of the uplink. The signal Q_(D) is filtered bylow pass filter 22 and is subsequently compared with a plurality ofthreshold values. The result of the comparison is used to divide theavailable channel capacity between the speech encoder 12 and the channelencoder 14. For the uplink and the downlink four different combinationsof the division of the channel capacity between the speech encoder 12and the channel encoder 14 are possible. These possibilities arepresented in the table below. TABLE 1 R_(x) R_(SPEECH)(kbit/s)R_(CHANNEL) R_(TOTAL)(kbit/s) 0 5.5 ¼ 22.8 1 8.1 ⅜ 22.8 2 9.3 {fraction(3/7)} 22.8 3 11.1 ½ 22.8 0 5.5 ½ 11.4 1 7.0 ⅝ 11.4 2 8.1 ¾ 11.4 3 9.3{fraction (6/7)} 11.4

[0022] From Table 1 it can be seen that the bitrate allocated to thespeech encoder 12 and the rate of the channel encoder increases with thechannel quality. This is possible because at better channel conditionsthe channel encoder can provide the required transmission quality (FrameError Rate) using a lower bitrate. The bitrate saved by the larger rateof the channel encoder is exploited by allocating it to the speechencoder 12 in order to obtain a better speech quality. It is observedthat the coding property is here the rate of the channel encoder 14. Thecoding property setting means 15 are arranged for setting the rate ofthe channel encoder 14 according to the coding property supplied by thesystem controller 16.

[0023] Under bad channel conditions the channel encoder needs to have alower rate in order to be able to provide the required transmissionquality. The channel encoder will be a variable rate convolutionalencoder which encodes the output bits of the speech encoder 12 to whichan 8 bit CRC is added. The variable rate can be obtained by usingdifferent convolutional codes having a different basic rate or by usingpuncturing of a convolutional code with a fixed basic rate. Preferably acombination of these methods is used.

[0024] In Table 2 presented below the properties of the convolutionalcodes given in Table 1 are presented. All these convolutional codes havea value ν equal to 5. TABLE 2 Pol/Rate 1/2 1/4 3/4 3/7 3/8 5/8 6/7 G₁ =43 000002 G₂ = 45 003 00020 G₃ = 47 001 301 01000 G₄ = 51 4 00002 101000G₅ = 53 202 G₆ = 55 3 G₇ = 57 2 020 230 G₈ = 61 002 G₉ = 65 1 110 02202000 000001 G₁₀ = 66  G₁₁ = 67  2 000010 G₁₂ = 71  001 G₁₃ = 73  010G₁₄ = 75  110 100 10000 000100 G₁₅ = 77  1 00111 010000

[0025] In Table 2 the values G_(i) represent the generator polynomials.The generator polynomials G(n) are defined according to:

G_(i)(D)=g₀⊕g₁·D⊕ . . . ⊕g_(n−1)·D^(n−1)⊕g_(n)·D^(n)  (A)

[0026] In (1) ⊕ is a modulo-2 addition. i is the octal representation ofthe sequence g₀, g₁, g_(ν−1), g_(ν). For each of the different codes thegenerator polynomials used in it, are indicated by a number in thecorresponding cell. The number in the corresponding cell indicates forwhich of the source symbols, the corresponding generator polynomial istaken into account. Furthermore said number indicates the position ofthe coded symbol derived by using said polynomial in the sequence ofsource symbols. Each digit indicates the position in the sequence ofchannel symbols, of the channel symbol derived by using the indicatedgenerator polynomial. For the rate ½ code, the generator polynomials 57and 65 are used. For each source symbol first the channel symbolcalculated according to polynomial 65 is transmitted, and secondly thechannel symbol according to generator polynomial 57 is transmitted. In asimilar way the polynomials to be used for determining the channelsymbols for the rate ¼ code can be determined from Table 3. The othercodes are punctured convolutional codes. If a digit in the table isequal to 0, it means that the corresponding generator polynomial is notused for said particular source symbol. From Table 2 can be seen thatsome of the generator polynomials are not used for each of the sourcesymbols. It is observed that the sequences of numbers in the table arecontinued periodically for sequences of input symbols longer than 1, 3,5 or 6 respectively.

[0027] It is observed that Table 1 gives the values of the bitrate ofthe speech encoder 12 and the rate of the channel encoder 14 for a fullrate channel and a half rate channel. The decision about which channelis used is taken by the system operator, and is signaled to the TRAU 2,the BTS 4 and the Mobile Station 6, by means of an out of band controlsignal, which can be transmitted on a separate control channel. 16. Tothe channel encoder 14 also the signal R_(U) is applied.

[0028] The block coder 18 is present to encode the selected rate R_(D)for transmission to the Mobile Station 6. This rate R_(D) is encoded ina separate encoder for two reasons. The first reason is that it isdesirable to inform the channel decoder 28 in the mobile station of anew rate R_(D) before data encoded according to said rate arrives at thechannel decoder 28. A second reason is that it is desired that the valueR_(D) is better protected against transmission errors than it ispossible with the channel encoder 14. To enhance the error correctingproperties of the encoded R_(D) value even more, the codewords are splitin two parts which are transmitted in separate frames. This splitting ofthe codewords allows longer codewords to be chosen, resulting in furtherimproved error correcting capabilities.

[0029] The block coder 18 encodes the coding property R_(D) which isrepresented by two bits into an encoded coding property encodedaccording to a block code with codewords of 16 bits if a full ratechannel is used. If a half rate channel is used, a block code withcodewords of 8 bits are used to encode the coding property. Thecodewords used are presented below in Table 3 and Table 4. TABLE 3 HalfRate Channel R_(D)[1] R_(D)[2] C₀ C₁ C₂ C₃ C₄ C₅ C₆ C₇ 0 0 0 0 0 0 0 0 00 0 1 0 0 1 1 1 1 0 1 1 0 1 1 0 1 0 0 1 1 1 1 1 1 1 0 1 1 1 0

[0030] TABLE 4 Full Rate Channel R_(D)[1] R_(D)[2] C₀ C₁ C₂ C₃ C₄ C₅ C₆C₇ C₈ C₉ C₁₀ C₁₁ C₁₂ C₁₃ C₁₄ C₁₅ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 0 1 1 1 1 0 1 0 0 1 1 1 1 0 1 1 0 1 1 0 1 0 0 1 1 1 1 0 1 0 0 1 1 1 11 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0

[0031] From Table 3 and Table 4, it can be seen that the codewords usedfor a full rate channel are obtained by repeating the codewords used fora half rate channel, resulting in improved error correcting properties.In a half-rate channel, the symbols C₀ to C₃ are transmitted in a firstframe, and the bits C₄ to C₇ are transmitted in a subsequent frame. In afull-rate channel, the symbols C₀ to C₇ are transmitted in a firstframe, and the bits C₈ to C₁₅ are transmitted in a subsequent frame.

[0032] The outputs of the channel encoder 14 and the block encoder 18are transmitted in time division multiplex over the air interface 10. Itis however also possible to use CDMA for transmitting the severalsignals over the air interface 10. In the Mobile Station 6, the signalreceived from the air interface 10 is applied to a channel decoder 28and to a further channel decoder being here a block decoder 26. Theblock decoder 26 is arranged for deriving the coding propertyrepresented by the R_(D) bits by decoding the encoded coding propertyrepresented by codeword C₀ . . . C_(N), in which N is 7 for the halfrate channel and N is 15 for the full rate channel.

[0033] The block decoder 26 is arranged for calculating the correlationbetween the four possible codewords and its input signal. This is donein two passes because the codewords are transmitted in parts in twosubsequent frames. After the input signal corresponding to the firstpart of the codeword has been received, the correlation value betweenthe first parts of the possible codewords and the input value arecalculated and stored. When in the subsequent frame, the input signalcorresponding to the second part of the codeword is received, thecorrelation value between the second parts of the possible codewords andthe input signal are calculated and added to the previously storedcorrelation value, in order to obtain the final correlation values. Thevalue of R_(D) corresponding to the codeword having the largestcorrelation value with the total input signal, is selected as thereceived codeword representing the coding property, and is passed to theoutput of the block decoder 26. The output of the block decoder 26 isconnected to a control input of the property setting means in thechannel decoder 28 and to a control input of the speech decoder 30 forsetting the rate of the channel decoder 28 and the bitrate of the speechdecoder 30 to a value corresponding to the signal R_(D).

[0034] The channel decoder 28 decodes its input signal, and presents ata first output an encoded speech signal to an input of a speech decoder30.

[0035] The channel decoder 28 presents at a second output a signal BFI(Bad Frame Indicator) indicating an incorrect reception of a frame. ThisBFI signal is obtained by calculating a checksum over a part of thesignal decoded by a convolutional decoder in the channel decoder 28, andby comparing the calculated checksum with the value of the checksumreceived from the air interface 10.

[0036] The speech decoder 30 is arranged for deriving a replica of thespeech signal of the speech encoder 12 from the output signal of thechannel decoder 20. In case a BFI signal is received from the channeldecoder 28, the speech decoder 30 is arranged for deriving a speechsignal based on the previously received parameters corresponding to theprevious frame. If a plurality of subsequent frames are indicated as badframe, the speech decoder 30 can be arranged for muting its outputsignal.

[0037] The channel decoder 28 provides at a third output the decodedsignal R_(U). The signal R_(U) represents a coding property being here abitrate setting of the uplink. Per frame the signal R_(U) comprises 1bit (the RQI bit ). In a deformatter 34 the two bits received insubsequent frames are combined in a bitrate setting R_(U)′ for theuplink which is represented by two bits. This bitrate setting R_(U)′which selects one of the possibilities according to Table 1 to be usedfor the uplink is applied to a control input of a speech encoder 36, toa control input of a channel encoder 38, and to an input of a furtherchannel encoder being here a block encoder 40. If the channel decoder 20signals a bad frame by issuing a BFI signal, the decoded signal R_(U) isnot used for setting the uplink rate, because it is regarded asunreliable.

[0038] The channel decoder 28 provides at a fourth output a qualitymeasure MMDd. This measure MMD can easily be derived when a Viterbidecoder is used in the channel decoder. This quality measure is filteredin the processing unit 32 according to a first order filter. For theoutput signal of the filter in the processing unit 32 can be written:

MMD′[n]=(1−α)·MMD[n]+αMMD′[n−1]  (B)

[0039] After the bitrate setting of the channel decoder 28 has beenchanged in response to a changed value of R_(D), the value of MMD′[n−1]is set to a typical value corresponding to the long time average of thefiltered MMD for the newly set bitrate and for a typical downlinkchannel quality. This is done to reduce transient phenomena whenswitching between different values of the bitrate.

[0040] The output signal of the filter is quantized with 2 bits to aquality indicator Q_(D). The quality indicator Q_(D) is applied to asecond input of the channel encoder 38. The 2 bit quality indicatorQ_(D) is transmitted once each two frames using one bit position in eachframe.

[0041] A speech signal applied to the speech encoder 36 in the mobilestation 6 is encoded and passed to the channel encoder 38. The channelencoder 38 calculates a CRC value over its input bits, adds the CRCvalue to its input bits, and encodes the combination of input bits andCRC value according to the convolutional code selected by the signalR_(U)′ from Table 1.

[0042] The block encoder 40 encodes the signal R_(U)′ represented by twobits according to Table 3 or Table 4 dependent on whether a half-ratechannel or a full-rate channel is used. Also here only half a codewordis transmitted in a frame.

[0043] The output signals of the channel encoder 38 and the blockencoder 40 in the mobile station 6 are transmitted via the air interface10 to the BTS 4. In the BTS 4, the block coded signal R_(U)′ is decodedby a further channel decoder being here a block decoder 42. Theoperation of the block decoder 42 is the same as the operation of theblock decoder 26. At the output of the block decoder 42 a decoded codingproperty represented by a signal R_(U)″ is available. This decodedsignal R_(U)″ is applied to a control input of coding property settingmeans in a channel decoder 44 and is passed, via the A-bis interface, toa control input of a speech decoder 48.

[0044] In the BTS 4, the signals from the channel encoder 38, receivedvia the air interface 10, are applied to the channel decoder 44. Thechannel decoder 44 decodes its input signals, and passes the decodedsignals via the A-bis interface 8 to the TRAU 2. The channel decoder 44provides a quality measure MMDu representing the transmission quality ofthe uplink to a processing unit 46. The processing unit 46 performs afilter operation similar to that performed in the processing unit 32 and22. Subsequently the result of the filter operation is quantized in twobits and transmitted via the A-bis interface 8 to the TRAU 2.

[0045] In the system controller 16, a decision unit 20 determines thebitrate setting R_(U) to be used for the uplink from the quality measureQ_(U). Under normal circumstances, the part of the channel capacityallocated to the speech coder will increase with increasing channelquality. The rate R_(U) is transmitted once per two frames.

[0046] The signal Q_(D)′ received from the channel decoder 44 is passedto a processing unit 22 in the system controller 16. In the processingunit 22, the bits representing Q_(D)′ received in two subsequent framesare assembled, and the signal Q_(D)′ is filtered by a first orderlow-pass filter, having similar properties as the low pass filter in theprocessing unit 32.

[0047] The filtered signal Q_(D)′ is compared with two threshold valueswhich depend on the actual value of the downlink rate R_(D). If thefiltered signal Q_(D)′ falls below the lowest of said threshold value,the signal quality is too low for the rate R_(D), and the processingunit switches to a rate which is one step lower than the present rate.If the filtered signal Q_(D)′ exceeds the highest of said thresholdvalues, the signal quality is too high for the rate R_(D), and theprocessing unit switches to a rate which is one step higher than thepresent rate. The decision taking about the uplink rate R_(U) is similaras the decision taking about the downlink rate R_(D).

[0048] Again, under normal circumstances, the part of the channelcapacity allocated to the speech coder will increase with increasingchannel quality. Under special circumstances the signal R_(D) can alsobe used to transmit a reconfiguration signal to the mobile station. Thisreconfiguration signal can e.g. indicate that a different speechencoding/decoding and or channel coding/decoding algorithm should beused. This reconfiguration signal can be encoded using a specialpredetermined sequence of R_(D) signals. This special predeterminedsequence of R_(D) signals is recognised by an escape sequence decoder 31in the mobile station, which is arranged for issuing a reconfigurationsignal to the effected devices when a predetermined (escape) sequencehas been detected. The escape sequence decoder 30 can comprise a shiftregister in which subsequent values of R_(D) are clocked. By comparingthe content of the shift register with the predetermined sequences, itcan easily be detected when an escape sequence is received, and which ofthe possible escape sequences is received.

[0049] An output signal of the channel decoder 44, representing theencoded speech signal, is transmitted via the A-Bis interface to theTRAU 2. In the TRAU 2, the encoded speech signal is applied to thespeech decoder 48. A signal BFI at the output of the channel decoder 44,indicating the detecting of a CRC error, is passed to the speech decoder48 via the A-Bis interface 8. The speech decoder 48 is arranged forderiving a replica of the speech signal of the speech encoder 36 fromthe output signal of the channel decoder 44. In case a BFI signal isreceived from the channel decoder 44, the speech decoder 48 is arrangedfor deriving a speech signal based on the previously received signalcorresponding to the previous frame, in the same way as is done by thespeech decoder 30. If a plurality of subsequent frames are indicated asbad frame, the speech decoder 48 can be arranged for performing moreadvanced error concealment procedures.

[0050]FIG. 2 shows the frame format used in a transmission systemaccording to the invention. The speech encoder 12 or 36 provides a group60 of C-bits which should be protected against transmission errors, anda group 64 of U-bits which do not have to be protected againsttransmission errors. The further sequence comprises the U-bits. Thedecision unit 20 and the processing unit 32 provide one bit RQI 62 perframe for signalling purposes as explained above.

[0051] The above combination of bits is applied to the channel encoder14 or 38 which first calculates a CRC over the combination of the RQIbit and the C-bits, and appends 8 CRC bits behind the C-bits 60 and theRQI bit 62. The U-bits are not involved with the calculation of the CRCbits. The combination 66 of the C-bits 60 and the RQI bit 62 and the CRCbits 68 are encoded according to a convolutional code into a codedsequence 70. The encoded symbols comprise the coded sequence 70. TheU-bits remain unchanged.

[0052] The number of bits in the combination 66 depends on the rate ofthe convolutional encoder and the type of channel used, as is presentedbelow in Table 5. TABLE 5 # bits/rate 1/2 1/4 3/4 3/7 3/8 5/8 6/7 Fullrate 217 109 189 165 Half rate 105 159 125 174

[0053] The two R_(A) bits which represent the coding property areencoded in codewords 74, which represent the encoded coding property,according the code displayed in Table 3 or 4, dependent on the availabletransmission capacity (half rate or full rate). This encoding is onlyperformed once in two frames. The codewords 74 are split in two parts 76and 78 and transmitted in the present frame and the subsequent frame.

1. Transmission system comprising a transmitter coupled via atransmission channel to a receiver, in which the transmitter comprises achannel encoder for encoding source symbols into coded symbols, and inwhich the receiver comprises a source decoder for deriving reconstructedsource symbols from the coded symbols received from the transmissionchannel, the transmission system comprising coding property settingmeans for setting at least one coding property of the channel encoderand the channel decoder, characterized in that the transmitter comprisesa further channel encoder for obtaining an encoded coding property fromthe at least one coding property, in that the transmitter is arrangedfor transmitting the encoded coding property to the receiver, in thatthe receiver comprises a further channel decoder for obtaining the atleast one coding property from the encoded coding property, and in thatthe receiver is arranged for setting the at least one coding property ofthe channel decoder according to the at least one coding property. 2.Transmission system according to claim 1, characterized in that thefurther channel encoder comprises a block encoder, and in that thefurther channel decoder comprises a block decoder.
 3. Transmissionsystem according to claim 1 or 2, characterized in that the encodedsymbols are arranged in frames, and in that the transmitter is arrangedfor splitting the encoded coding property into at least two parts, andfor transmitting said at least two parts in subsequent frames. 4.Transmission system according to one of the previous claims,characterized in that the transmission system comprises transmissionquality determining means for deriving a transmission quality measurefrom the channel decoder in the receiver, and means for transmitting viaa further transmission channel the quality measure to the transmitter.5. Transmitter comprising a channel encoder for encoding source symbolsinto coded symbols and property setting means for setting at least onecoding property of the channel encoder characterized in that thetransmitter comprises a further channel encoder for obtaining an encodedcoding property from the at least one coding property, and in that thetransmitter is arranged for transmitting the encoded coding property. 6.Receiver comprising a source decoder for deriving reconstructed sourcesymbols from received coded symbols, and coding property setting meansfor setting at least one coding property of the channel decodercharacterized in that the receiver comprises a further channel decoderfor obtaining at least one coding property from an received encodedcoding property, and in that the receiver is arranged for setting the atleast one coding property of the channel decoder according to the atleast one coding property.
 7. Integrated circuit comprising a channelencoder for encoding source symbols into coded symbols and propertysetting means for setting at least one coding property of the channelencoder characterized in that the integrated circuit comprises a furtherchannel encoder for obtaining an encoded coding property from the atleast one coding property, and in that the integrated circuit isarranged for transmitting the encoded coding property.
 8. Integratedcircuit comprising a source decoder for deriving reconstructed sourcesymbols from received coded symbols, and coding property setting meansfor setting at least one coding property of the channel decodercharacterized in that the integrated circuit comprises a further channeldecoder for obtaining at least one coding property from an receivedencoded coding property, and in that the integrated circuit is arrangedfor setting the at least one coding property of the channel decoderaccording to the at least one coding property.
 9. Transmission methodcomprising encoding source symbols into encoded symbols and transmittingthe encoded symbols via a transmission channel, receiving encodedsymbols from the transmission channel, deriving reconstructed sourcesymbols from the encoded symbols received from the transmission channeland setting at least one coding property of the channel encoder and thechannel decoder, characterized in that the method further comprisesobtaining an encoded coding property from the at least one codingproperty, transmitting the encoded coding property via the transmissionchannel, receiving the encoded coding property from the transmissionchannel, obtaining the at least one coding property from the encodedcoding property, and setting the at least one coding property of thechannel decoder according to the at least one coding property. 10.Method comprising encoding source symbols into coded symbols and settingat least one coding property, characterized in that the method comprisesobtaining an encoded coding property from the at least one codingproperty, and in that the method comprises transmitting the encodedcoding property.
 11. Method comprising deriving reconstructed sourcesymbols from received coded symbols, and setting at least one codingproperty characterized in that the method comprises obtaining at leastone coding property from an received encoded coding property, and inthat the method comprises setting the at least one coding property ofthe channel decoder according to the at least one coding property.